Catheter delivery systems are commonly used to introduce self-expanding endoprostheses in human or animal bodies and to advance them to the clogged or narrowed area. In the delivery system, the elongate endoprosthesis is held in a radially compressed state by a surrounding sheath to facilitate a smooth delivery. When the endoprosthesis has been placed in the destined area, it is expanded by withdrawing or opening up the sheath.
A catheter delivery system where the endoprosthesis is expanded by cutting open the sheath is disclosed in FR 2688688, which is incorporated by reference into this application. In this system, three cutting wires are arranged equidistantly around the periphery of the endoprosthesis. Each wire runs from a proximal end of the catheter to a distal end, with the wire placed between the radially compressed endoprosthesis and the sheath in the region where the endoprosthesis is received, leaves the sheath at its distal end and runs back to the proximal catheter end along the outside of the sheath, so as to form a loop around the sheath wall. Both parts of the wires, in- and outside the sheath, are guided parallel to one another and the overall six proximal wire ends are attached to a handle at the proximal end of the catheter. The sheath is opened by pulling the handle so that the distal ends of the three wire loops move proximally and cut through the wall of the sheath.
A catheter delivery system that uses only one cutting wire is disclosed in WO-A-01/08599, which is incorporated by reference into this application. The wire consists of an inner pull element, running within the sheath, an outer pull element, running outside the sheath, and a separating element, located between the distal ends of the two pull elements at the distal end of the sheath. In order to expand the endoprosthesis, both pull elements are simultaneously pulled in a proximal direction, so that the separating element moves along the endoprosthesis towards the proximal catheter end and cuts through the sheath wall. Also belonging to the state of the art is WO2004/066809, which is incorporated by reference into this application, and which suggests the use of a deployment line that is integral with a pull back sheath, to release an endoluminal device from inside the sheath.
In known catheter delivery systems that use a cutting mechanism to open up the sheath, the cut open sheath is trapped between the expanded endoprosthesis and the wall of the vessel, once the expansion process is finished. To remove the sheath from the patient's body, it has to be pulled out from its proximal end. For the case of relatively large endoprostheses, such as oesophagus stents, where sheaths with thick walls can be used, this procedure is normally uncomplicated. However, problems arise when small-sized endoprostheses are required, for example to widen narrow blood vessels. In this case, the profile of the distal catheter end, comprising the endoprosthesis to be deployed, has to be strongly reduced, in order to facilitate accurate placement of the endoprosthesis and thus sheaths with thin walls have to be used. When such a thin-walled, cut open sheath is removed from the patient's body by pulling from its proximal end, the friction generated by the abluminal surface of the expanded endoprosthesis and the luminal surface of the vessel may cause either the sheath to tear, inhibiting its complete removal, or the endoprosthesis to move proximally with the sheath being pulled away from the axial position in the bodily lumen where it ought to be. Similar friction problems may arise even in traditional deployment methods, where an unslitted sheath is withdrawn from the endoprosthesis in the expansion process. When pulled from the proximal end, a thin-walled sheath may stretch along the direction of the pull, leading to a decrease of its radial diameter. This increases the friction caused between sheath and endoprosthesis, requiring a larger pulling force to move the sheath, similar to the known concept of the “Chinese finger trap”. Eventually, the sheath may tear or the endoprosthesis may move away from the desired position.
While one way to release a stent from the constraint of a sheath is to run a slitter down the length of the sheath that surrounds the stent (see, for example, WO 01/08599, WO 93/17636 and WO 98/20812, each of which is incorporated by reference into this application), a far more commonly used method is to pull the sheath proximally, relative to the stent, to release the stent progressively, starting at its distal end. See, for example, U.S. Pat. No. 5,833,694 Poncet, which is incorporated by reference into this application. However, pulling on the sheath from its proximal end will impose tensile stresses on the sheath material, in the length direction of the sheath. The sheath must therefore be physically strong enough, not only to resist the radially outward spring force that is coming from the self-expanding stent constrained within the sheath, but also the stresses to which the sheath is subject during the time that it is being pulled proximally, to release the stent. A force that tends to stretch a tube will also tend to reduce the diameter of that tube. A simple example is the toy called the “Chinese finger” that is in the form of a braided tube. The child slips it over his finger and then pulls the free end of the tube, causing the braiding to neck down, and grip tightly the finger within the lumen of the braided tube. Intuitively, one can appreciate that pulling on a sheath from the proximal end, when there is a self-expanding stent inside the lumen of the sheath, is a situation fraught with the possibility of unwanted additional, friction-generated loads that might be an impediment to smooth release of the stent.
One can appreciate that the wall thickness of the sheath makes a contribution to the totality of the passing profile of the stent delivery system, at least at the distal end of the catheter delivery system, in the “capsule” that contains the stent to be delivered by that system, for any such capsule is liable to be that portion of the length of the catheter that exhibits the greatest passing diameter. It is normally possible to devise a catheter delivery system in which the length of the catheter, the shaft, between the proximal end of the catheter held in the hand of the operator, and the distal end that exhibits the capsule containing the stent, will be of smaller passing diameter. Thus, designers are interested to reduce the wall thickness of the sheath surrounding the self-expanding stent, to the extent that is possible, consistent with safe operation of the system. It is of course intolerable for the sheath component to fail during deployment of the stent. The sheath component must be robust enough to withstand less than optimal handling of the stent delivery system, by medical practitioners, during placement of the stent.
It is an object of the present invention to advance stent delivery system design to achieve a smaller passing profile, consistent with simplicity of design and reliability of operation.